The short answer: unplug it, detach the controller and cord completely, and wash it alone on a short gentle cold cycle with mild detergent. The heating wires are sealed inside the blanket and water does not reach them, which is why most modern blankets are machine washable at all. What damages them is mechanical: wringing, twisting, folding along the same creases, and clothes pegs clamped onto the fabric. Never wring it and never iron it. Dry it flat rather than hung, because the weight of the water pulls on the wiring, and make sure it is completely dry before reconnecting the controller. And check the connector: one maker says to fold the blanket so the connector block sits wrapped in the middle, away from the drum wall.
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